1. Field of the Invention
The present invention is concerned with improved overbased copper fungicidal spray compositions containing an alkaline earth metal to reduce the phytotoxicity thereof.
As is well known most of the parasitic diseases of plants are caused by fungi. Practically every species of plant is more or less subject to attack by parasitic fungi. Fungi cause the greatest damage to crops.
The fungicidal agents most often used are sulphur, mercury, certain organic compounds and cooper. Copper fungicides have been known for several centuries. Just after the turn of the 17th Century the Abbe Prevost showed that fungous spores were prevented from germinating when they have been in contact with a solution of copper sulphate. However, water soluble copper compounds cannot be used on plants because the cupric ion is extremely toxic to plant life. Sparingly soluble derivatives of copper must be used for leaf application. Not all water insoluble compounds of copper are fungicidal and it is believed that the in vitro fungicidal activity of a copper compound is dependent largely on its solubility in spore exudate or other solubilizing substance associated with the plant itself. The mechanism by which copper enters solution from the dried, alkaline, well-known Bordeaux mixture is believed to be one of complex formation. It seems that the fungicidal action of cooper depends upon reaction in the fungal cell. Relatively little attention seems to have been given to oil soluble (water insoluble) derivatives of copper.
Since copper fungicides are usually insoluble in water they are applied in the form of powders or in relatively large volume aqueous suspensions and as such are readily removed by wind and rain necessitating applications at short intervals which is expensive.
It was suggested previously that this disadvantage might be overcome by using a non-phytotoxic carrier oil containing an oil soluble copper compound. In particular it was suggested that a spray oil in which the copper is solubilized as copper naphthenate would possess improved resistance to rain and wind, better contacting, and be cheaper.
A marketable clear fungicidal spray oil has yet to be developed. A fungicidal copper spray with increased activity and high resistance to rain is claimed in a patent to Farb. Bayer A.-G. (O. Telle and F. Grewe, Ger. Pat. No. 1,088,285/1957; Chem. Abs., 1962, 57, 5161i), the copper compound being mixed with magnesium soaps and fatty acid esters. A clay-oil emulsion of copper naphthenate is claimed (O. N. Yuganova, Chem. Abs., 1951, 45, 5351g) to stick to foliage. Certain petroleum oils used as supplements to fixed-copper spray formulations greatly improve the performance of the copper fungicide, the oil acting in part to increase the weathered copper deposit on the surface of the sprayed leaves (J. D. Wilson, Plant Disease Reptr., 1961, 45, 282; J. D. Wilson and O. K. Hedden, ibid, 1963, 47, 564; J. D. Wilson, The Pesticide News, Ohio, 1962, 15, 56). Oil-based copper fungicides in low volume were found (S. Screenivasan, Chem. Abs., 1969, 71, 37889p) superior to Bordeaux mixture and, by their ability to stick to the leaf, they resisted rain washing. The preparation of oil soluble copper xanthates for use as fungicides has been described by S. B. Tuwiner (U.S. Pat. No., 2,651,649/1953, to Phelps Dodge Corp.; Chem. Abs., 1954, 48, 8819d). The copper compounds suggested by these researchers are not economical enough to warrant widespread use. The less expensive copper naphthenate, usually in kerosine, has been used as a wood preservative (P. I. Smith, Soap, 1938, 14(11), 86; Chem. Abs., 1939, 33, 777.sup.7 ; B. C. Bera, Chem. Abs., 1963, 59, 3004a). Complex copper salts of sulphurized cresylic and naphthenic acids are claimed (A. J. Krus, U.S. Pat. No. 2,188,951, to Richards Chemical Works, Inc., Chem. Abs., 1949, 34, 3872.sup.9) for use as fungicides for trees and plants. Copper naphthenate paste has been suggested (N. V. Ptitsyna, et al., Chem. Abs., 1962, 56, 12033g; 1964, 61, 1196e) as a substitute for Bordeaux mixture.
It has been found, however, that neutral copper naphthenate carried in a non-phytotoxic carrier oil caused severe leaf burn when applied to grapefruit albeit at relatively large dosage and concentration. In co-assigned patent application Ser. No. 360,506 it is disclosed that overbased copper naphthenates in which the ratio of metal to naphthenic acid ranges from over 1 to 20 carried as a clear dispersion in spray oil are less phytotoxic because of the in-built buffer effect of the excess base. When applied to Valencia orange leaves at 0.1% w/v copper and at a dosage of 12 gal./acre (low severity) the spray oil solution of a nominal 1000% overbased copper naphthenate was found to be almost completely nonphytotoxic. However, when tested at a higher concentration of copper (1% w/v) and higher application level (250 gal./acre, high severity) the 1000% overbased copper naphthenate spray oil caused severe leaf burn. This result was unexpected since neither zinc nor manganese overbased naphthenates had been found phytotoxic at the high severity applications. It was noted that the severity of leaf burn caused by the foliar sprays increased with increasing concentration of the overbased copper naphthenate. Thus the phytotoxicity of the overbased copper naphthenate spray oil was found to depend on the concentration of copper in the spray oil as well as the quantity of oil applied. In contrast to overbased zinc and manganese naphthenates the overbased copper naphthenate spray oil applied at high severity dosage is definitely phytotoxic. At low severity but still practical dosages the spray oil is relatively non-phytotoxic to citrus. But it is evident that a spray oil containing overbased copper naphthenate alone would be restricted to a low concentration of copper.